The present invention retates in general to a laser beam printer. In particular, the present invention concerns control of a laser beam in the laser beam printer.
A general arrangement of the laser beam printer to which the invention is directed is illustrated in FIG. 1. In the case of the illustration, a laser device 12 is constituted by a semiconductor laser or the like which is capable of generating a modulated laser light beam. A light-sensitive drum 11 is rotated by a motor 17 in the direction indicated by an attached arrow. The laser beam modulated in accordance with information or data to be printed as is reflected by a polyhedral mirror 13 which is rotated by means of a motor 14, whereby the drum surface of the rotating light-sensitive drum 11 is scanned on a line-by-line base with the modulated laser light beam to produce latent images of patterns such as characters to be printed. An example of such scanning system is disclosed in U.S. Pat. No. 3,750,189. The latent images are subsequently developed with toner and transferred to a recording sheet. The development process is classified into a negative process and a positive process. In the case of the former, only those portions which are irradiated with light are developed by toner. To the contrary, according to the positive process, those portions which are not irradiated with light are developed in black while the portions undergone no irradiation remaining in blank.
More specifically, in the case of the positive development process, the peripheral surface of the photosensitive drum is uniformly charged with positive charge. When the drum surface is irradiated selectively with the modulated laser beam, electric charge on those portions of the drum surface which are subjected to the irradiation is annulled, while electric charge on the non-irradiated areas or locations remains as it is. Toner particles (i.e. developer) are charged with negative charge and adhere to those portions of the drum surface where the electric charge remains without having been erased. The toner image or images thus formed are transferred onto a recording sheet.
FIG. 2 illustrates fragmentally the drum surface of the light-sensitive drum 11 in the process of the positive development. In this figure, reference numeral 16 designates a single line scan (or a single raster line).
Symbol A represents areas or regions irradiated with light, while symbol B represents an area or location which has not been irradiated because of the laser beam having been interrupted due to the modulation. By the way, it is noted that the scanning line tends to be deviated upwardly or downwardly or takes a meandering path in dependence on the precision with which the polyhedral mirror is finished and/or distortions of the rotation axis. FIG. 3 illustrates a case in which an overlying scanning line has meandered along a curved path, whereby a zone B which has not been irradiated is produced and makes appearance on the recording sheet in a scratch-like form in black. Such failed line scan brings about a smear in the background of the printed record distinctly to an eyesore, particularly in the case of the positive development, involving degradation in the record or print quality.